SentiWordNet is a lexical resource for opinion mining. SentiWordNet
assigns to each synset of WordNet three sentiment scores: positivity,
negativity, objectivity. SentiWordNet is described in details in the
papers:

How to obtain SentiWordNet

The current "official" version of SentiWordNet is 3.0, which is based on WordNet 3.0.

SentiWordNet is distributed under the Attribution-ShareAlike 3.0 Unported (CC BY-SA 3.0) license. Among the other possibilities, this license allows the use of SentiWordNet in commercial applications, provided that the application mentions the use of SentiWordNet and SentiWordNet is attributed to its authors.